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Full Discussion: Making sure only one instance of a script is running
Top Forums Shell Programming and Scripting Making sure only one instance of a script is running Post 302989678 by rbatte1 on Tuesday 17th of January 2017 07:02:01 AM
Old 01-17-2017
I use this block of code early in scripts that need to be run separately:-
Code:
# Set up / test lock file.  Hold lock for whole script
lockfile="/var/lock/my-lock-file"
[ ! -f $lockfile ] && touch $lockfile
exec 13>> $lockfile

flock -w 30 -x 13                                           # Do you feel locky?

if [ $? -ne 0 ]
then
   echo "$0 bypassed due to lock" >> $tracefile
   logger "$0 bypassed due to lock"
   exit 666
else
   echo "$0 has the the lock" >> $tracefile
   logger "$0 has the the lock"
fi

Naturally, I use a suitable file name for the lock file so that it is easy to recognise and re-use/avoid for other scripts. It seems to work for me. The tracefile variable and other things have already been set.

The -w flag on flock means that it will wait on file descriptor 13 for 30 seconds trying to get an exclusive lock (the -x flag) The file descriptor is left open by the >> redirector so the lock persists.

If the process is killed off ungracefully (i.e. a kill -9) then the file is then closed and the lock released, else it is closed when the script terminates gracefully or can be closed earlier if that is appropriate with exec 13>&- It negates the need for the trap ....... 0 which leaves you free to use it for something else if that is appropriate.


As they say, Tim Toady



Robin
This User Gave Thanks to rbatte1 For This Post:
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LEARN ABOUT LINUX

service

service(8)						      System Manager's Manual							service(8)

NAME

       service - run a System V init script

SYNOPSIS

       service SCRIPT COMMAND [OPTIONS]

       service --status-all

       service --help | -h | --version

DESCRIPTION

       service runs a System V init script in as predictable environment as possible, removing most environment variables and with current working
       directory set to /.

       The SCRIPT parameter specifies a System V init script, located in /etc/init.d/SCRIPT.  The  supported  values  of  COMMAND  depend  on  the
       invoked	script,  service  passes  COMMAND and OPTIONS it to the init script unmodified.  All scripts should support at least the start and
       stop commands.  As a special case, if COMMAND is --full-restart, the script is run twice, first with the stop command, then with the  start
       command.

       service --status-all runs all init scripts, in alphabetical order, with the status command.

EXIT CODES

       service calls the init script and returns the status returned by it.

FILES

       /etc/init.d
	      The directory containing System V init scripts.

ENVIRONMENT

       LANG, TERM
	      The only environment variables passed to the init scripts.

SEE ALSO

       /etc/init.d/skeleton,
       update-rc.d(8),
       init(8),
       invoke-rc.d(8).

								     Jan 2006								service(8)
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